The present invention relates to opto-electronic devices, and more specifically, to read out of quantum states of microwave frequency qubits with optical frequency photons.
In one approach called circuit quantum electrodynamics, quantum computing employs nonlinear superconducting devices called qubits to manipulate and store quantum information at microwave frequencies, and resonators (e.g., as a two-dimensional (2D) planar waveguide or as a three-dimensional (3D) microwave cavity) to read out and facilitate interaction among qubits. As one example, each superconducting qubit may comprise one or more Josephson junctions shunted by capacitors in parallel with the junctions. The qubits are capacitively coupled to 2D or 3D microwave cavities. The electromagnetic energy associated with the qubit is stored in the Josephson junctions and in the capacitive and inductive elements forming the qubit.
In one example, to read out the qubit state, a microwave signal is applied to the microwave readout cavity that couples to the qubit at the cavity frequency corresponding to the qubit state A. Transmitted (or reflected) microwave signal goes through multiple thermal isolation stages and low-noise amplifiers that are required to block or reduce the noise and improve the signal to noise ratio. The microwave signal is measured at room temperature. High microwave signal indicates that the qubit is in state A. Microwave readout provides a stable signal amplitude for control, and commercial off-the-shelf (COTS) hardware is available to use that covers most of microwave frequency ranges. However, microwave readout signal must be isolated from thermal noise and amplified with minimum noise added.
Quantum systems such as superconducting qubits are very sensitive to electromagnetic noise, in particular in the microwave and infrared domains. In order to protect these quantum systems from microwave and infrared noise, several layers of filtering, attenuation, and isolation are applied. Of particular interest are the layers of protection employed on the input and output (I/O) lines, also called transmission lines, that are connected to the quantum system, and carry the input and output signals to and from the quantum system respectively. In the case of superconducting qubits, these I/O lines (transmission lines) are usually microwave coaxial lines or waveguides. Some of the techniques or components that are used in order to block or attenuate the noise propagating or leaking into these transmission lines are attenuators, circulators, isolators, low-pass microwave filters, bandpass microwave filters, and infrared filters which are based on lossy absorptive materials. However, these noise isolation components and microwave signal amplification techniques require a large amount of additional microwave hardware and cost.